Many industries have a commercial need to monitor the concentration of particular constituents in a fluid. The oil refining industry, wineries, and the dairy industry are examples of industries where fluid testing is routine. In the health care field, there are numerous examples of the need to monitor a particular constituent within bodily fluids. A number of systems are available that allow testing of body fluid, such as, blood, urine, or saliva, to conveniently monitor the level of a particular fluid constituent, such as, for example, cholesterol, proteins, and glucose. Patients suffering from diabetes, a disorder of the pancreas where insufficient insulin production prevents the proper digestion of sugar, must carefully monitor their blood glucose levels on a daily basis. A number of systems that allow people to conveniently monitor their blood glucose levels are available. Such systems typically include a test strip where the user applies a blood sample and a meter that “reads” the test strip to determine the glucose level in the blood.
Among the various technologies available for measuring blood glucose levels, electrochemical technologies are particularly desirable because only a very small blood sample may be needed to carry out the measurement. In electrochemical-based systems, the test strip typically includes a sample chamber that contains reagents, such as glucose oxidase and a mediator, and electrodes. When the user applies a blood sample to the sample chamber, the reagents react with the glucose, and the meter applies a voltage to the electrodes to cause a redox reaction. The meter measures the resulting current and calculates the glucose level based on the current.
In a typical electrochemical sensor, regardless of the size of the system, the oxidation or reduction half-cell reaction involving glucose either produces or consumes electrons. This electron flow can be measured, provided the electrons can interact with a working electrode that is in contact with the sample to be analyzed. The electrical circuit is completed through a counter electrode that is also in contact with the sample. A chemical reaction occurs at the counter electrode, and this reaction (oxidation or reduction) is the opposite of the reaction at the working electrode. See, for example, Fundamentals Of Analytical Chemistry, 4th Edition, D. A. Skoog and D. M. West; Philadelphia: Saunders College Publishing (1982), pp 304-341.
Another feature of some conventional miniaturized electrochemical test strips is the presence of a single layer of biological reagents over both the working and counter electrodes. The components of this reagent layer include the enzyme that facilitates the oxidation-reduction reaction of glucose, a coenzyme, and any mediators or other substances that help to transfer electrons between the oxidation-reduction reaction and the working electrode. The use of a single reagent layer can provide for simple manufacturing of the strips, since only one deposition step is needed to coat the material onto the electrodes. However, due to the biological nature of the reagent layer, it is difficult to reproducibly manufacture each strip with the exact same sensitivity. In addition, most reagent compositions exhibit vastly different electrochemical properties. The best reagent compositions are those that exhibit increased sensitivity by facilitating the free flow of electrons between the sample being analyzed and the electrode and its connected circuitry.
Accurate measurement of blood glucose levels may be critical to the long-term health of many users. As a result, meters and test strips used to measure blood glucose levels should be highly reliable. However, as sample sizes become smaller, the dimensions of the sample chamber and electrodes in the test strip also become smaller. This, in turn, may make test strips more susceptible to smaller manufacturing defects and to damage from subsequent handling.
Accordingly, the present invention provides improved biological reagent compositions with improved sensitivity to the concentration of blood glucose in patient samples for use in measuring systems and methods, whereby a relatively constant linear relationship exists between the measured current and the glucose concentration for the time scale of the analysis.